I have developed my plugin for Chrome.
But, I'd like to develop for Vivaldi.
Where is the Document?

There is no published documentation for Vivaldi extensions yet. So far, Vivaldi is able to use many Chrome extensions, but only so long as no toolbar buttons or UI interaction such as that is required. After Vivaldi has implemented extensions (and they have not yet), would be the time to start looking for documentation on how to build them.